The advent of computers using page description languages (PDL) has had a dramatic impact on the printing and publishing industry. PDLs describe an image in the form of a computer-readable list of geometric, textual, and other visual elements, which are referred to collectively as "objects". PDLs have permitted the wide spread growth of computer-based publishing and printing into traditional printing areas including the manufacturing of high quality process color images.
High quality process color images are normally made on multiple pass color printing systems. There are several different techniques used for multiple pass color printing systems, but most start with a plurality of monochromatic images of the output color image that each hold the percent tint of a single hue, commonly referred to as a separation color or separation. The monochromatic images, which used to consist of black and white images on a photographic film or media, contained all the information about the combination of colors used to make the output color image. In the past, these photographic films were used to produce a plurality of corresponding etched metallic plates which in turn carried the specific single hue or separation for the output color image. The output color image being formed by the multiple printings of the etched plates carrying their various hues or separations.
Multiple pass color printing systems commonly use either a three or four color separation system, with the most common system being the four component cyan, magenta, yellow and black (CMYK) color separation system. However there can be a greater or lesser number of separations depending on the effects in the image that the end-user desires. It is not is uncommon to have 6 to 10 separations for a very high quality output image.
The introduction of PDLs and computer-based printing produced a change in the techniques used in process color printing. Rather than using monochromatic images on photographic films, a plurality of PDL separation files are now used to describe the separations. This change from film to computer-readable files has permitted new changes in the process color print industry that were not available before, such as the ability to quickly alter and edit the shape of an image electronically and the ability to directly etch the metallic plates from the information contained in the PDL separation files.
Furthermore the ability to easily edit and alter the colors in an image in the form of a page description language has also lead the emergence of digital preproofing techniques. In the large volume print industry, digital preproofing techniques save money on production costs by proofing and editing an output color image from proofs obtained from a proofing device other than the actual production printer.
The proofing devices currently in use in digital preproofing systems are usually a combination of a set of color conversion techniques that convert from one separation system to another separation system, such as CMYK to RGB (Red, Green & Blue), and displayed the image from the output from a color printers, or a display on cathode ray tube (CTV) display unit. These color conversion techniques are well-know in the art and are explained by various publications, such as the PostScript.RTM. Language Reference Manual, 2nd ed., by Adobe Systems Incorporated, Addison-Wesley Publishing Company (hereinafter referred to as the "RedBook") at Ch.2.2 pp. 12-13.
However, current digital preproofing techniques do not permit the objects described in the PDL separation files to be combined into a single display list of objects for these proofing devices. Furthermore, the move to PDL-based systems also created another problem that did not exist in the traditional film-based techniques. All PDL display lists not only contain information about the visual relationship of the objects to one another in the two dimensions, or x- and y-axis, but also in the foreground to background ordering of objects, or z-axis. Accordingly, the ordering of the objects in a display list, or z-order, becomes critically important in the producing the correct image. For example, if the display list contains objects in the wrong z-order a background object could be erroneously placed too early in the order, with the resulting image having a misplaced background object producing an undesirable visual error in the image that would obscure or overlap foreground objects.
For PDL separation files, the z-order of the objects in the display list creates an additional problem in producing a single "combined" display list. When combining the PDL separation files, problems about the conflicts in object z-orders occur between the different PDL separation files. One of the principal reasons why this problem occurs is that an object will only occur in a PDL separation file if that an object contributes the color that PDL separation file represents. For example, in a CMYK system if the output color image had a blue object, which is a combination of cyan and magenta, the blue object would not be listed as an object in the yellow and black separation files because the blue object does not contribute any color to the yellow and black separations. If the output color image also contained a red object, i.e. a combination of magenta and yellow, and a yellow object, then the PDL separation file representing the magenta would be able to define the order of the red object in relation to the blue object since both objects share the common color of magenta. But the order of the blue object in relation to the yellow object could not be determined from any single PDL separation file as the yellow object and blue object do not share any common separation colors. Nevertheless, the problem of the relative relationship between the blue and yellow objects can be solved by comparing the order of the objects between the different PDL separation files, i.e. the order of the yellow object relative to the blue object can be determined from the order of the blue object to the red object in the "magenta" PDL separation file, and red object to the yellow object in the "yellow" PDL separation file.